Corneal Topography
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What is Corneal Topography
Corneal topography is a technology that digitally analyzes corneal morphology through a computer image processing system. It can express the obtained information in color morphological maps with different characteristics. These morphological maps are similar to the undulations of the ground surface in geography, so they are called corneal topography. This technology can measure the corneal refractive power from the center to the periphery of the cornea, thereby obtaining more information.
Advantages of Corneal Topography
Large measurement area and large amount of information obtained
The corneal curvature meter can only measure 8% of the total area of the cornea, while the corneal topography can observe more than 95% of the range, the density of data points can be as high as 34 rings, with 256 points per ring, the entire cornea can have 7000-8000 data points, and some of them can reach tens of thousands of data points.
Wide range of refractive power measurement
It can accurately measure the refractive power of corneas that are too flat or too steep.
High accuracy and small error
Cornea 8.0mm range accuracy of 0-0.07D, due to the use of instant digital video technology in 1/30s display, to avoid the impact of transient eyes and heartbeat.
Easy to establish mathematical model
Due to the use of raster photogrammetry technology, it is easy to establish mathematical models because the spherical subtraction map with relative and height signs and the corneal meridian curvature sign map use height points instead of curvature to explain the changes of the corneal surface.
Less affected by corneal pathology (compared to keratoconus)
New corneal topographers such as PAR and CTS can not only examine the cornea with epithelial defects, ulcers and scarring, but also the results are seldom affected by corneal lesions, so the results are of high reference value.
Visualization of results
Due to the use of color, different colors are used for different radii of curvature on the cornea. The warm color represents the part with strong refractive power, and the cold color represents the part with weak refractive power, which makes the results shown in corneal topography very intuitive and eye-catching.
Multi-purpose
Corneal topographer also has the function of automatic corneal curvature meter and keratoscope, and the new corneal topographer can also measure the pupil diameter and corneal diameter under visual and dark vision. The function of its software has also been greatly improved, such as through the set function conversion, you can get the information of corneal aberration and so on.
Corneal topography works by measuring the light reflected off the cornea. It utilizes a computerized system that shines a series of concentric rings of light onto the cornea. The shape the light rings take when reflected back is captured and analyzed by a computer, creating a detailed map of the corneal surface.
The resulting map, often color-coded for easier interpretation, reveals minute details about the cornea's shape and curvature. Red areas on the map typically indicate steeper, or higher areas, while blue areas indicate flatter, or lower areas. This detailed information provides a comprehensive understanding of the cornea’s condition, enabling eye health professionals to make accurate diagnoses and treatment plans.
Corneal topography is used in a variety of contexts within eye care. It is commonly used to diagnose and manage a range of corneal conditions, such as keratoconus, a condition where the cornea thins and bulges into a cone-like shape. By creating a detailed map of the cornea, corneal topography can detect subtle changes in the cornea's surface, providing early diagnosis of conditions like keratoconus.
Additionally, corneal topography is indispensable in contact lens fitting, especially for patients with irregular corneas. The detailed map allows for a more accurate and comfortable fit, significantly improving the patient’s overall experience.
Types Of Corneal Topography
Placido disc topography
Placido disc topography systems focus on the anterior cornea. They gather information about tear film quality, irregularities, curvature, and foreign bodies. The system relies heavily on the tears that reflect the light it produces. The systems can either be large or small cone designs.
Eye doctors prefer the small cone because it collects more detail and data points. However, large cones are much easier to use, and collecting data is easier too.
Scanning slit-Lamp and scheimpflug topography
These corneal topography systems provide information for the posterior and anterior cornea. They detect and manage corneal swelling. They are useful when the eye doctor deals with contact lens wearers who are affected more by this condition.
What Are The Common Topography Maps From Corneal Topography?
Axial display map
The axial display map is the most conventional technique for viewing the topography image. It focuses mainly on corneal power. Despite being the most conventional, eye doctors consider it less accurate. This conception arises from the fact that it produces a smooth map by collecting data averages. It determines the base curvature of a soft contact lens.
Tangential display map
It is more accurate than the axial map because it provides accurate measurements of corneal curvature and power. Because of this, it is vital in fitting contact lenses like ortho-k lenses. Ophthalmologists can also use it to measure the power of a contact lens when it is on the eye. This feature is necessary when fitting multifocal lenses with different powers and meridians.
When a cornea is healthy, its curvatures are even, and its surface is very smooth. These are important in helping the eye focus light on the back of the eye. The cornea accounts for about 70 percent of the eye's focusing power.
Corneal topography will reveal any irregularities in the shape of the cornea. Irregularities in the corneal surface may indicate disease, injury, or other conditions. An eye doctor can use the procedure for any of the following reasons:
• Monitoring, treating, or diagnosing conditions affecting the cornea.
• Evaluation of corneal deformities or injuries.
• Planning surgery for refractive errors.
• Treating and evaluating astigmatism.
• To determine the type of intraocular lenses for cataract surgery.
• To track eye health after surgery.
• Detecting keratitis, which is very infectious.
• Assessing aspects affected by glaucoma.
• Examining parts of the cornea.
Enter the patient's name, age, sex, and diagnosis into the computer after turning on the computer.
Explain the examination process to the patient and keep the patient comfortable during the examination. The patient should take a sitting position, put the lower jaw on the mandibular tray, and use the headband to fix the head position.
Ask the patient to look at the fixed light in the center of the keratoscope. This light varies from machine to machine and may be? This light varies from machine to machine and may be continuous or flashing, and may be red or green. It may be red or green.
The examiner operates the handle of the corneal topographer so that the cross point on the display screen is located in the center of the pupil, i.e., so that the center point of the concentric circles of the corneal lens coincides with the center point of the pupil and the focus is adjusted so that the concentric circles of the Placido disk on the display screen have a clear image, and then the button is pressed so that the image is fixed. Before photography, the patient should be instructed to blink several times to make the reflection of the eye surface uniform. During photography, the patient should be instructed to keep both eyes open at the same time. This can be done several times per patient to select the best image for analysis. This procedure? A critical step in corneal topography and should be practiced before the examination.
The examiner chooses to display the corneal image as needed. In addition to the coded corneal topography image, the display screen of the image contains additional information about the image, which can be analyzed by the physician to evaluate the patient's anterior corneal surface.
Soft lenses should be removed for at least 2 weeks; hard lenses should be discontinued for at least 4 weeks.
The head position and eye position of the examined person should be correct, and should not be tilted, otherwise it may cause axial change of corneal astigmatism, etc. 3. both eyes should be opened fully.
Keep both eyes wide open to fully expose the cornea, but avoid pressure on the cornea.
Keep the corneal surface moist, for those with unstable tear film, artificial tears can be put in before the examination, so as to avoid drying of the cornea and affecting the test results.
If the patient's facial shadow is found to affect the examination, the patient can be instructed to tilt slightly toward the examined eye to avoid facial shadow and make the examination more accurate. If the patient has ptosis, another person can be asked to assist in lifting the upper lid, but be careful not to compress the eyeball.
Factors Affecting the Results of Corneal Topography Examination
The camera must be centered and well-focused before taking the corneal image, otherwise a corneal camera error will result in an asymmetrical or irregular cornea. Otherwise, corneal camera errors can result in asymmetrical or irregular corneal topography.
Since the corneal camera takes an air-tear film topography, this requires an intact tear film. Excessive tears can build up underneath the cornea, creating the illusion of a localized steepening of the cornea underneath on the topographic map.
A dry corneal surface and an incomplete tear film will result in a localized flattening of the corneal surface, which can be resolved by having the patient blink repeatedly or applying artificial tears prior to the exam.
Patients with ptosis or senile laxity of the upper eyelid often need to lift the upper eyelid with their hands during corneal topography, which often compresses the eyeball or pulls the eyeball excessively, and will also cause changes in corneal topography.
Commonly Used Parameters in Corneal Topography
Commonly used parameters in corneal topography are Surface Asymmetry Index (SAI), Surface Regularity Index (SRI), Analog Keratoscopic Reading, Minimum Keratoscopic Reading, and Corneal Predictive Visual Acuity (CPVA), as described below:
Surface Asymmetry Index (SAI): It is the sum of the difference between the two corresponding refractive errors in each of the 10 rings 180° apart. Theoretically, the normal cornea near the central region is similar to the spherical surface, refractive power is highly symmetrical distribution, SAI should be close to 0 less than 0.3. China's normal eye is 0.3 ± 0.1. SAI value is greater indicates that the corneal surface is more irregular, when the cornea is highly asymmetric (such as cone cornea), SAI can be up to 5.0 or more.
Surface regularity index (SRI): Is the surface regularity within 10 rings. Theoretically, it should be close to 0. The smaller the SRI value is, the more regular the corneal surface is, which is 0.2±0.2 for normal people in China.
Simulated keratoscope reading (SimK): For the meridian of the maximum refractive power in the 7th, 8th, 9th ring on the average, as well as 90 ° from the meridian direction of the same 3 ring average, while marking the axial direction.
Minimum keratoconus reading (MinK): Is the average of the 7th, 8th, and 9th rings on the minimum refractive power meridian and the axial direction.
Predicted visual acuity (PVA): The visual acuity that can be obtained from a cornea when the refraction, retina, optic nerve, and refractive interstitium of the eye are normal; PVA correlates well with SAI and SRI, and a comparison of PVA with the patient's actual corrected visual acuity can be used to distinguish whether or not the visual dysfunction is corneal in origin.
Components of Corneal Topography
Placido disc projection system 16 to 34 concentric rings are uniformly projected onto the corneal surface from the center to the periphery, the diameter of the center ring can be as small as 0.4mm, and the rings can cover the entire cornea.
Instant Image Camera System The ring image projected on the corneal surface can be instantly observed, monitored and adjusted by the instant image camera system to keep the corneal image in the best condition, and then photographed with a digital video camera and stored in a computer for analysis and processing.
Computer Image Processing System The computer first digitizes the stored images, then applies the pre-set calculation formulas and procedures to analyze them, and then displays the different analysis results on the display screen with different colored images (in color), and at the same time, the analyzed statistics are also displayed together, and can be printed through the connected color printer.
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VisualTech (Shanghai) Corporation, commonly referred to as VisualTech, is a professional supplier of ophthalmic equipment, optometry and ophthalmic medical instruments, as well as optical lab equipment for the eyeglasses. The company is located in the bustling metropolis of Shanghai, China.
VisualTech is committed to innovative self-development, constantly striving to lead the industry while adhering to the highest standards of quality. The company provides customers with high-quality, innovative and trustworthy services.
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